Head exposure to cellular phones: a system level study

8/8/2019 Head exposure to cellular phones: a system level study

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Head Exposure

to Cellular Telephones:A System-Level Study

Hayat Abdulla and Renny E. Badra

Departamento de Electrnica y Circuitos - Universidad Simn Bolvar

Caracas, Venezuela


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Why a system-level study? It can be stated that the question of potential risks

associated to the use of cell phones close to usersheads is still open.

So far, prevention efforts have been focused oncontrolling the output levels of cell phones.

Little attention has been paid to impact of systemparameters, such as technology and quality of coverage,

among others.

System parameters are mainly affected by cellularoperators.


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SAR and transmit power Effect of non-ionizing radiation over organic tissue is

heat dissipated from EM emissions.

SAR (Specific Absortion Rate) is defined as the amountof power dissipated per unit mass of tissue [W/kg]. For regulatory purposes, SAR is obtained experimentally. Maximum FCC tolerated head SAR level is 1.6 W/kg

(source: ANSI).

This work asumes a linear relationship between SAR andaverage transmit power:

SAR = PtK

SAR


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Experiment DesignDetermine statisticsof transmit power

Obtain statistics ofSAR

Evaluate impact ofsystem parameters

Monte Carlo Simulations (approx.260.000 trials per cell).

Link power budget obtained usingstatistical propagation models.

Power control algorithms applied. Parameter KSAR obtained for each cell

phone model under study.

Linear relationship between SAR andaverage power applied.

Effect of system parameters on twoindicators (average SAR throughoutcell and percentage of trials above0.16 W/kg) is evaluated.


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Link Budget Analysis

Pmin

= Rsen +Lp + Lii

Gii

Pt = Power_Control(Pmin )

Minimum RequiredTransmit Power

Receiver Sensitivity

Radio ChannelPropagation Loss

Other power lossesalong signal path

Power gains alongsignal path

Actual Transmit Power Application of power control algorithm


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Link Budget ParametersPARAMETER GSM CDMA2000 1X UMTS

(Eb/No)req N/A 3.0 dB 5.0 dB

SNRreq 9.0 dB N/A N/A

Receiver Noise Figure 5.0 dB

Receiver Sensitivity -107.0 dBm -126.6 dBm -123.1 dBm

BS Antenna Type Sectorized, 90 beamwidth

BS Antenna Max Gain 15.1 dBi

BS Cable losses 2 dB

Co-channel interference 2.0 dB N/A N/A

Uplink Load Factor N/A 3.0 dB (50% load)

10log() N/A 1.5 dB 1.8 dBMS ant. gain + cable loss 0 dB

BS ant. diversity gain 3.0 dB Included in (Eb/No)req

Voice Activity Factor 70% 67% 67%

Soft Handoff Gain N/A Random variable between 0 and 5 dB


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Radio Channel ModelCOMPONENT MODEL

Distance-dependent propagation loss Walfisch-Ikegami model for

LOS and NLOS locations

Shadowing loss Zero-mean log-normal random variable with

8 dB standard deviation

Small scale fading Rayleigh (NLOS) or Rician (LOS) amplitude

distributions

Structure penetration loss Random Variable uniformly distributed

between 0 and 12 dB (NLOS locations only)

Human body loss Constant value of 3 dB

LOS/NLOS discrimination Random variable with standard probability

distribution as a function of distance


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Power Control Parameters

Parameter GSM 1X UMTS

Maximum Power 33 dBm 25 dBm 23 dBm

Minimum Power 5 dBm -50 dBm -50 dBm

Granularity 2 dB 1 dB 1 dB


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Cell phone parametersGSM Model1 Model2 Model3 Model4 Model5 Average

SAR [w/kg] 0.840 0.776 0.660 1.24 0.476 0.798

[dBm] 23.97 23.22 22.92 22.97 20.06 22.42

KSAR [kg-1] 3.368 3.698 3.409 6.259 5.912 4.529

1X Model1 Model2 Model3 Model4 Model5 Average

SAR [w/kg] 1.160 0.721 1.090 1.130 0.728 0.996

[dBm] 23.30 24.87 25.07 25.20 25.64 24.82

KSAR [kg-1] 5.426 2.349 3.392 3.413 1.988 3.314

UMTS Model1 Model2 Model3 Model4 Model5 Average

SAR [w/kg] 1.06 0.876 1.00 1.08 0.706 0.944

[dBm] 23.00 22.05 23.70 23.60 22.94 23.06

KSAR [kg-1] 5.313 5.464 4.266 4.714 3.588 4.669


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Results for 90%-confidence urban

cellGSM Model1 Model2 Model3 Model4 Model5 Average

A v e rageSAR [w/kg]

0.073 0.080 0.074 0.136 0.128 0.098

F[%] 13 13 13 19 19 15.4



0.125 0.068 0.098 0.124 0.072 0.098

F[%] 19 11 16 19 13 15.6



0.098 0.101 0.079 0.087 0.066 0.086

F[%] 16 16 13 13 11 13.8

Cell size: 1150 m (GSM), 1750 m (1X), 1260 m (UMTS)


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Results for 2-km urban cellGSM Model1 Model2 Model3 Model4 Model5 Average


0.131 0.143 0.132 0.243 0.229 0.176

F[%] 26 26 26 34 34 29.2



0.160 0.081 `0.117 0.138 0.080 0.115

F[%] 25 13 20 21 15 18.8


A v e rage

SAR [w/kg]

0.162 0.166 0.130 0.144 0.109 0.142

F[%] 29 29 24 24 20 25.2

Coverage confidence: 70% (GSM), 86% (1X), 75% (UMTS)


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Fraction of Monte Carlo trials in

which cell phones transmit atmaximum power

Fixed 90% coverage

Fixed cell radius

Urban Suburban Urban (2 km) Suburban (5km)

GSM 4% 4% 8% 9%

1X 3% 3% 4% 4%

UMTS 3% 3% 6% 7%


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Conclusions We have quantified the level of head emissions from

cellular phones operating under the three majortechnologies, as a function of system parameters.

Novel methodology based on linearly relating SAR andtransmit power.

SAR levels depend on coverage confidence levels of thecell more strongly than on any other system parameter

Higher confidence reduce SAR levels.

Improving the reverse link budget design margins translatesnot only into higher coverage reliability but also into lowertransmit power levels and lower head emissions.


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Conclusions Thus, enhancing the uplink coverage tend to have a

possitive impact in the average emission levels on thehead of cellular voice users.

Techniques such as increased cell density, higher cell towers,the use of cell tower amplifiers, cell antenna diversity,microcells and active repeaters are recommended.

When comparing different cell phone models, all otherfactors kept constant, significant differences in averageSAR levels have been found: up to almost a 2:1 ratiowithin the same technology.


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Head Exposure

to Cellular Telephones:A System-Level Study

Hayat Abdulla and Renny E. Badra

Departamento de Electrnica y Circuitos - Universidad Simn Bolvar

Caracas, Venezuela

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Head exposure to cellular phones: a system level study